Some of the most horrifying terrorist attacks of all time from 9/11 to Mumbai, these murderous acts killed several thousands of people and caused millions of pounds of damage to properties worldwide.

The Wall Street Bombing on 16th September 1920 immediately killed 30 people from the blast, with others dying later from their injuries and wounding 143, the cause of the bombing was never solved

Mr Morrell was speaking in Washington alongside Michael Vickers, former under secretary of defence for intelligence, and former US ambassador to Syria Robert Ford.

The respected trio of Washington officials said ISIS was now more dangerous than ever.

The FBI is thought to have more than 900 open investigation into homegrown terrorists in the US and more than 36,000 fighters have fled to Syria to join the group, with around 6,600 believed to have Western passports.

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Police and sniffer dogs on a security sweep in New York

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US president Barack Obama has warned of the threat

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Syed Rizwan Farook was behind a massacre in California

Mr Ford said he feared 'ISIS 2.0' would emerge as an even stronger force if, after the group was defeated in the coming years, efforts were not then made to bring political stability to Syria.

He said: "In order to mobilise Sunni Arabs to contain the Islamic State, there must be efforts at national reconciliation.

"This is important because we don't want the Islamic State to be put down militarily and then revive, as happened between 2011 and 2013.

"I really don't want to see an Islamic State, version 2.0."

Mr Vickers added that the existing military campaign against the warped regime was "a fraction of what it should be" and called for a renewed push to remove Syrian president Bashar al-Assad from power.